1. Field of the Invention
The present invention relates to a seamless tube with a grooved inner surface preferable for a heat transfer tube to be incorporated in an air-cooled heat exchanger employed in air conditioner for home or commercial use and to a method and apparatus for producing the same. More particularly, it relates to a seamless tube with a grooved inner surface formed by a roll rolling process with excellent productivity and to a method and apparatus for producing the same.
2. Description of Related Art
As an example of a method for producing a seamless tube with a grooved inner surface, there is one using a rolling process as disclosed in JP-B No. 1991-5882. A description will be given herein below to a method for producing a seamless tube with a grooved inner surface using a roll rolling process (hereinafter referred to as a roll rolling method). As a raw material, a raw tube made of an unannealed material (H-material) is used. A grooved plug having a groove formed in the outer surface thereof is inserted into the raw tube, while a pair of rolls rotating in contact with the outer surface of the raw tube are disposed at positions corresponding to the grooved plug. The axes of rotation of the rolls are adjusted to be orthogonal to the axis of the raw tube. Each of the rolls is configured as a hollow type having a diameter larger at the center portion thereof than at the end portions thereof such that a cross-sectional configuration including the axis of rotation of the roll generally matches the configuration of the outer surface of the raw tube in a cross section orthogonal to the axis of the raw tube. By drawing the raw tube, while pressing the tube against the grooved plug by means of the rolls, the grooved plug rotates around the tube axis and the groove in the grooved plug is transferred to the inner surface of the raw tube so that a groove is formed therein.
In the roll rolling method, the grooved plug forms a groove only in a local region of the inner surface of the tube so that a load on the groove plug is smaller than in a ball rolling method. Therefore, it is also possible to form a groove parallel to the axial direction of the tube in the outer circumferential surface of the grooved plug and form a groove parallel to the axial direction of the tube in the inner surface of the tube.
However, the roll rolling method has the problem that a groove is not formed evenly in the entire inner surface of the tube. The groove is formed in the inner surface of the portion of the tube pressed by the rolls but is not formed in the inner surface of the portion of the tube not pressed by the rolls.
To solve the problem, the JP-B No. 1991-5882 discloses technology using a plurality of roll pairs each composed of two rolls such that the tube is pressed by the pairs or rolls in different directions. This allows even formation of a groove in the entire inner surface of the tube, as described in the publication.
However, the foregoing conventional technology has the following problem. In accordance with the method for producing a tube with a grooved inner surface disclosed in the JP-B No. 1991-5882, a groove is formed in the inner surface of the tube by pressing the tube in a specified direction by means of a pair of rolls and then a groove is formed in the inner surface of the tube by pressing the tube in a direction different from the foregoing specified direction by means of another pair of rolls. In this case, the following phenomenon occurs. FIGS. 19A to 19C and FIGS. 20A to 20C are cross-sectional views illustrating a conventional method for producing a seamless tube with a grooved inner surface, which are orthogonal to the tube axis. It is to be noted that the process steps shown in FIGS. 19A to 19C and FIGS. 20A to 20C are performed continuously by the same apparatus.
FIG. 19A is a cross-sectional view showing a configuration of the tube in a cross section orthogonal to the tube axis before groove formation. As shown in FIG. 19A, the tube 51 before groove formation has a generally circular cross-sectional configuration and no groove is formed in an inner surface. Then, as shown in FIG. 19B, a grooved plug 52 is inserted into the tube 51, while the tube 51 is pressed by a pair of rolls 53 being rotated outside the tube 1 in contact relation therewith. Consequently, the inner surface of the tube 51 is partly formed with a groove 59, while the tube is flattened in the direction in which it is pressed, as shown in FIG. 19C.
Next, as shown in FIG. 20A, a grooved plug 54 is inserted into the tube 51, while the tube 51 is pressed by a pair of rolls 55 being rotated outside the tube 1 in contact relation therewith. The rolls 55 press the tube 51 in a direction orthogonal to the direction in which the rolls 53 press the tube 51. Since the tube 51 has been flattened in the direction in which it is pressed by the rolls 53, a large clearance 56 is formed between the grooved plug 54 and the tube 51.
If the tube 51 is pressed by the rolls 55 in this state, a groove 60 is formed in the inner surface of the tube 51, as shown in FIG. 20B. At that time, protrusions 57 are formed at the portions of the tube 51 corresponding to the clearance between itself and the rolls 55. If a deep groove 60 is to be formed, the protrusions 57 are increased in size. If the tube 51 formed with the protrusions 57 is then subjected to a sizing process using a die or the like, the protrusions 57 are retracted into the tube so that depressed portions 58 are formed disadvantageously. As a result, the tube 51 no more has a smooth outer surface so that the commercial value of the tube 51 is reduced significantly. In FIG. 20C, the grooves 59 and 60 are not depicted.
Thus, in accordance with the method for producing a seamless tube with a grooved inner surface by performing a roll rolling process using plural pairs of rolls with respect to the tube disclosed in the JP-B No. 1991-5882, it is impossible to produce a seamless tube with a grooved inner surface which has a sufficiently high quality as a commercial product.
The present invention has been achieved in view of the foregoing problem and it is therefore an object of the present invention to provide a method for producing a seamless tube with a grooved inner surface which allows a seamless tube with a grooved inner surface having a smooth outer surface and an inner surface formed with different kinds of grooves to be produced at low cost and with high productivity, while preventing the formation of the protrusions.
A method for producing a seamless tube with a grooved inner surface according to the present invention comprises the steps of: drawing a metal tube to gradually reduce a diameter of the metal tube by means of a holding die disposed externally of the metal tube and a holding plug disposed internally of the tube and engaged with the holding die; rotating a plurality of first rolls in contact with an outer surface of the metal tube with a reduced diameter such that a rotation axis of each of the first rolls is orthogonal to an axial direction of the metal tube, while disposing, at a position inside the metal tube corresponding to the first rolls, a first grooved plug formed with a grooved outer surface and coupled in relatively rotatable relation to the holding die via a first coupling shaft, and pressing the metal tube against the first grooved plug by means of the first rolls to form a first groove in a portion of an inner surface of the metal tube in a circumferential direction of the tube and thereby form a plurality of first grooved zones; performing a sizing process with respect to the metal tube formed with the first grooved zones by using a sizing device; and rotating a plurality of second rolls in contact with the outer surface of the metal tube such that a rotation axis of each of the second rolls is orthogonal to the axial direction of the metal tube and deviates from the rotation axis of each of the first rolls, while disposing, at a position inside the metal tube corresponding to the second rolls, a second grooved plug formed with a grooved outer surface and coupled in relatively rotatable relation to the first grooved plug via a second coupling shaft, and pressing the metal tube against the second grooved plug by means of the second rolls to form a second groove in a portion of the inner surface of the metal tube in the circumferential direction of the tube and thereby form a plurality of second grooved zones.
In the present invention, the provision of the step of performing the sizing process with respect to the metal tube between the step of forming the metal tube with the first grooved zone and the step of forming the metal tube with the second grooved zone corrects the configuration of the metal tube that has been pressed and flattened by the first rolls so that the configuration of the metal tube in a cross section orthogonal to the tube axis becomes generally circular. Consequently, the clearance between the second grooved plug and the metal tube is minimized in the step of forming the second grooved zone. This prevents the formation of a protrusion at the portion of the metal tube corresponding to the space between the second rolls. As a result, a depressed portion resulting from the protrusion is not formed in the subsequent diameter reducing step and a seamless tube with a grooved inner surface having a smooth outer surface can be formed. In addition, the first grooved zone can be formed only in a portion of the inner surface of the metal tube as a result of pressing of the metal tube by means of the first rolls and then the second grooved zone can be formed in a region different from the first grooved zone as a result of pressing the metal tube by means of the second rolls. This allows the formation of a groove pattern other than a simple spiral groove in the inner surface of the metal tube.
Each of the first and second grooves may be formed in a region of the inner surface of the metal tube located between the first and second grooved zones. The arrangement allows the formation of a third region formed with each of the first and second grooves in the region located between the first and second grooved zones.
Alternatively, a groove need not be formed in a region of the inner surface of the metal tube located between the first and second grooved zones. If the seamless tube with a grooved inner surface is used as a heat transfer tube, the arrangement allows the suppression of a pressure loss in a refrigerant flowing through the tube.
Preferably, an even number of first rolls and an even number of second rolls are provided and disposed in mutually opposing relation with the metal tube interposed therebetween. For example, each of the numbers of the first rolls and the second rolls may be two. This allows a force with which one of the rolls presses the metal tube to be received by the other of the rolls opposing the roll. Accordingly, processing can be performed efficiently and the strength of the producing apparatus can be increased.
Preferably, each of respective directions in which the grooves formed in the first and second grooved plugs extend is parallel to the axial direction of the tube or inclined at an angle of 0 to 30xc2x0 to the axial direction of the tube. This allows the formation of a groove parallel to the axial direction of the tube or a groove inclined to the axial direction of the tube in the inner surface of the tube. By adjusting the angle between the groove and the axial direction of the tube to less than 30xc2x0, a load applied to the grooved plug can be reduced and a loss in the grooved plug can be prevented. Since the tube is elongated in the process of reducing the diameter, the angle of the groove decreases after the diameter is reduced. The amount of the decrease of the angle depends on the ratio of reducing the diameter. The angle between the groove and the axial direction of the tube at the time of forming grooves should be about 30xc2x0 in order to obtain the angle of 20xc2x0 in the final stage, in a usual ratio of reducing the diameter.
Preferably, respective circumferential speeds of at least one of the first rolls and the second rolls are adjusted to be higher than a speed at which the metal tube is drawn. The arrangement achieves a reduction in frictional force produced between the first or second roll and the metal tube as well as a reduction in the drawing stress of the metal tube, thereby preventing the rupture of the metal tube reliably.
The method for producing a seamless tube with a grooved inner surface can be used preferably and appropriately if the metal is composed of copper or a copper alloy. In this case, a proof stress of the metal tube composed of copper or a copper alloy before the grooves are formed in the metal tube is preferably 200 to 500 N/mm2 and a proof-stress/tensile-strength ratio of the metal tube before the grooves are formed in the metal tube is preferably 0.65 to 0.95.
By thus using the raw tube having a proof stress and a proof-stress/tensile-strength ratio falling within the foregoing ranges, the raw tube is prevented from being excessively elongated, excellent producibility is provided, and the rupture of the raw tube can be prevented, while the processing speed can be increased with the application of a large drawing force. This allows a tube with a grooved inner surface to be obtained with high productivity at low production cost. The proof stress defined herein is a 0.2% proof stress.